Installation for producing a nonwoven web with very uniform weight

ABSTRACT

The invention concerns an installation for producing a nonwoven web wherein the distance between the lower edge of the slit drawing device and the upper edge of the diffuser is maintained constant by uniformly distributed hasps.

BACKGROUND OF THE INVENTION AND RELATED ART

The present invention relates to installations for producing nonwovenwebs.

An installation for producing a nonwoven web is already known thatcomprises, in succession from the top down, a generator of a curtain offilaments, especially plastic filaments, a slot attenuator forattenuating the filaments of the curtain followed, at a distance fromthe lower edge of the attenuating slot attenuator, by a diffuser and aconveyor for collecting the filaments exiting the diffuser.

The generator of a curtain of filaments usually comprises an extruderintended to extrude a molten organic polymer through a spinneret drilledwith numerous holes so as to form a curtain of filaments and, beneaththe spinneret, a device for cooling the curtain of filaments. The slotattenuator for attenuating the filaments of the curtain generally hasopposed sidewalls and opposed endwalls that define an oblong inlet slotfor receiving the filaments and an oblong outlet slot from which thefilaments exit. A slot-shaped passage extends between the inlet and theoutlet and the filaments pass through it, being attenuated by theinjection of a stream of air into the slot-shaped passage, which streamis sufficient to attenuate the filaments. Beneath this slot attenuatorfor attenuating the filaments of the curtain is the diffuser, which isintended to spread out the incoming curtain. Since the diffuser divergesor is flared downward, the curtain that passes through it progressivelyspreads out as it falls. The web that thus forms on the collectingconveyor placed beneath the diffuser should be more uniform.

It turns out in practice that this uniformity as defined for example bythe weight per m² of the web is not achieved as well as would be liked.

SUMMARY OF THE INVENTION

The invention remedies this drawback by an installation of the typeabove in which the web formed is more uniform.

According to the invention, the distance between the lower edge of theattenuating slot attenuator and the upper edge of the diffuser isconstant to within ±7%, and preferably to within ±5%.

It was realized in fact, unpredictably, that it is of primary importanceas regards the uniformity of the web for the distance between the loweredge of the attenuating slot attenuator and the upper edge of thediffuser to be approximately constant.

However, this had not been achieved hitherto since the oblong slot edgesin question usually have their long side with lengths exceeding 150 cm,which sag or deform under the effect of the actual weight of theattenuator and of the diffuser.

According to the invention, means are therefore provided for keeping thelower edge of the attenuating slot attenuator, and also preferably theupper edge of the diffuser, horizontal. The means are such that theyleave the gap between the lower edge of the attenuating slot attenuatorand the upper edge of the diffuser clear, but they prevent these edgesfrom sagging or curving inward. These means may be formed by the factthat the attenuator and the diffuser are, at least along the edges inquestion, made of a very rigid material.

According to one particularly preferred embodiment, the means comprisebridges uniformly distributed along the perimeter of the lower edge andof the upper edge, the branches of which bridge are fixed to theattenuator at points closer to its lower edge than its upper edge and tothe diffuser at points closer to its upper edge than its lower edge.These bridges or other means for keeping the edges horizontal, which maybe provided every 200 to 300 mm, ensure horizontality of the edges andtherefore constancy of their mutual separation, without, however,requiring the use of very rigid and therefore expensive materials.

The uniformity of the web is also improved if at least one slotextending over the entire perimeter of the diffuser and means forkeeping the lower and upper edges of this slot horizontal are providedalong the diffuser, it being possible for these means again to bebridges such as those used to keep the gap between the attenuating slotattenuator and the diffuser constant.

The distance between the upper edge of the diffuser and the lower edgeof the attenuator is generally between 3 mm and 5 mm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective schematic view of an installation according tothe invention.

FIG. 2 is a sectional view of the diffuser.

FIG. 3 is a perspective partial view showing a bridge fixed to theattenuating slot attenuator and to the diffuser, and

FIGS. 4 and 5 are graphs illustrating the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

The installation comprises an extruder 1 fed with a synthetic polymerand a spinneret 2 for forming a curtain of filaments 3. The spinneret isformed from a plate having numerous holes with a diameter that dependson the filaments extruded. These holes are distributed over a number ofparallel rows. For example, there are 18 rows over a spinneret width of140 mm.

At the exit, that is to say just below the spinneret 2, there is acooling unit 4 for lowering the temperature of the filaments andcomposed of a number of successive zones 4 a, 4 b, 4 c, which allows thecurtain of filaments 3 to be subjected to streams of air whose velocityand temperature may be adjusted. The length of this cooling zone may bearound 1200 mm.

Downstream, and therefore beneath this cooling unit 4, there is aconventional attenuator 5 with a slot F. It is composed of two wallsthat define between them a passage in the form of a slot F, into whichpressurized air, for example at a pressure of 0.5 bar, is injected. Thisslot attenuator makes it possible to suck the curtain of filaments andentrain it by high-velocity air streams, thereby attenuating thefilaments.

Mounted below the slot attenuator 5 is a diffuser 6. This diffuser 6,shown in particular in FIG. 2, has two walls 14, 15 making an angle of5° between them and each being provided with three openings 16 extendingover the entire length. The diffuser 6 is placed 3 mm below theattenuator 5 and the width d of the attenuation slot is just less thanthe width D of the top of the passage defined by the diffuser 6. Mountedin the wall 15 of the diffuser 6 is the electrostatic device 17.

There is a conventional conveyor 7 beneath the diffuser 6.

The distance between the lower edge 20 of the attenuating slotattenuator and the upper edge 21 of the diffuser is constant to within5%, being 3 mm∓0.15 mm. Screwed into the top of the diffuser is thelower branch 22 of a bridge 23, the upper branch 24 of which is screwedinto the bottom of the attenuating slot attenuator. The central part ofthe bridge 23 is at a distance of at least 1 cm from the gap between thelower edge 20 and the upper edge 21 so that this gap remains entirelyclear.

FIG. 4 is a graph showing the variation in the distance between the edge20 and the edge 21, while FIG. 5 corresponds to the variation in theweight in g/m² of the web deposited on the conveyor as a function ofthese distances.

In FIG. 4, the y-axis shows the distances in mm between the lower edgeof the attenuator and the upper edge of the diffuser and the x-axisshows the widths of the web in mm along these edges, while in FIG. 5 thewidths of the web in mm are plotted on the x-axis and the weight in g/m²of the web is plotted on the y-axis. These show that, for the widthx=200, a still acceptable weight in g/m² is obtained, but the slope ofthe weight in g/m² curve changes suddenly with the sudden increase inweight below x=200 mm, whereas the slope of the curve representing thedistance between the two edges is constant between approximately 100 mmand 300 mm.

1. An installation for producing a nonwoven web comprising, insuccession from the top down, a generator of a curtain of filaments, acooling unit for cooling the curtain, a slot attenuator for attenuatingthe filaments of the curtain, said slot attenuator having a lower edgefollowed, at a distance from the lower edge of the attenuating slotattenuator, by a diffuser with an upper edge and a conveyor, wherein agap between the lower edge of the slot attenuator and the upper edge ofthe diffuser is unobstructed and the distance between the lower edge ofthe attenuating slot attenuator and the upper edge of the diffuser isconstant to within +5%, and further including means for keeping theedges horizontal and spaced apart, said means being laterally spacedfrom the edges and extending between the attenuating slot attenuator andthe diffuser at spaced locations along the edges.
 2. The installation asclaimed in claim 1, characterized in that the means comprise bridgesuniformly distributed along the perimeter of the lower edge and of theupper edge, the branches of which bridges are fixed to the attenuator atpoints closer to its lower edge than its upper edge and to the diffuserat points closer to its upper edge than its lower edge and the centralpart of the bridges are laterally at a distance of at least 1 cm fromthe gap.
 3. The installation as claimed in claim 1, characterized inthat means for keeping the edges horizontal are provided every 200 to300 mm along the edges.